In a transceiver such as those used as so called base stations, i.e. a station which has a controlling function for the users in a cell, it is important to be able to calibrate the transmitter and the receiver, in particular in cellular systems of the TDD kind, Time Division Duplex. In particular, many base stations can have multiple transmitters and receivers, which will create a need to calibrate all of these transmitters and receivers. This need is particularly important in so called TDD systems within the LTE concept, Long Term Evolution. In LTE, the base stations are usually referred to as eNodeB.
One known way of carrying out calibration of transmitters and/or receivers in an eNodeB is so called design or factory calibration. As the name implies, this kind of calibration is done at the manufacturing or design stage, and puts high demands on accuracy in design and manufacturing, as well as a great need for accurate information on variations which need to be taken into account.
If this method is used, there will be no information about site specific factors such as, for example, feeders and jumpers between the base station and an antenna input, which means that such information needs to be given to, for example, a central processor which can use the information later as a correction basis for misalignments in the system.
Another known method for calibration is to calibrate time and/or/phase differences by means of a calibration unit. According to this method, the calibration unit is used to calibrate the transmitter(s) and/or the receiver(s). Generally speaking, calibration of, for example, a transmitter, is carried out by means of the unit injecting a known signal into the transmitter at baseband level, feeding the signal back from a suitable point, and comparing this signal with the known injected signal, by means of which it is possible to calculate variations in such parameters as, for example, gain and phase/delay. Based on such calculations, a correction or compensation can then be made.
Such an extra unit will be quite large, and will need to be installed in the base station. The unit will be powered more or less constantly, and will create additional system noise even in an ideal mode. This method also causes a need for extra cables to be connected between the sampling point(s) and the calibration unit, as well as other cables to transfer sampled signals for further processing in the baseband part of the base station.